Textbooks and teachers often present information in a way that
suggests that what they say is fact. In the case of the mass of
a football, every source I found had the same numerical value
for the mass of a football. According to the Oxford Companion
to World Sports and Games, the official National Football
League Encyclopedia, the NFL "make the right call"
book and the football "fun facts"sheet the mass of
a football must be between 14 and 15 ounces (0.40 to 0.43 kg)
for it to be used in a NFL game.

Football evolved from rugby which evolved from soccer. When
the game of football (American) was established all that was
official was the shape of the ball, it looked like "an elongated
pumpkin"which was later called a prolate spheroid. In 1912
the weight of a football was established to be 14 to 15 ounces
(0.40 to 0.43 kg)

Before each professional football game all twenty-four balls
(if the game is played in a domed stadium) or thirty-six balls
(if the game is played in an open-air stadium) must be inspected
by the referee. This is done by sight, making sure that each ball
is a Wilson ball signed by the NFL commissioner. The ball size
and shape are also tested by placing each ball in a mold which
each referee has.

For the most part football is the one sport I have never been
able to understand.